1. Field of the Invention
The present invention relates to an automatic output power controller wherein signals modulated by a base band signal accompanied by variation of amplitude are output in gain adjustment so that mean power of output signals of an amplifier amplifying the modulated signals becomes constant.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an automatic output power controller in the prior art. In FIG. 1, numeral 1 designates a base band signal, numeral 2 designates an oscillator for generating a carrier 2a, numeral 3 designates a modulator, numeral 4 designates a high frequency signal (hereinafter referred to as "RF signal") being a modulated wave, numeral 5 designates an RF power amplifier with a gain adjusting terminal, numeral 6 designates an amplifier RF output signal, numeral 7 designates an RF output power detector using a coupler and a diode, numeral 8 designates a differential amplifier as voltage difference output means for comparing a detected value with reference voltage, numeral 9 designates a low pass filter (hereinafter referred to as "LPF"), a numeral 10 designates a feedback control signal.
Next, operation will be described.
A carrier 2a is subjected to prescribed modulation by a base band signal 1 in a modulator 3. A modulated wave 4 obtained in this manner is input to an RF power amplifier 5, and is amplified to the required power for transmission. The transmission power must be within a definite allowable range even when source voltage, input power or the like is varied. Consequently, the output power must be always detected and feedback must be applied so as to suppress the variation. In this case, first, the power of the output signal 6 is detected as voltage by an RF output power detector 7 comprising a coupler and a PIN diode as above described, and difference between the detected voltage and the reference voltage is obtained in a differential amplifier 8. The difference signal obtained in this manner passes through an LPF 9 in order to improve the noise resistant property and to remove influence of the amplitude variation attendant upon the modulation and is made a feedback control signal 10. The feedback control signal 10 is applied to the gain adjusting terminal of the RF power amplifier 5. The RF power amplifier 5 increases the gain when the output level as a whole falls, and decreases the gain when the output level rises, so that the output of constant level can be obtained.
Since the automatic output power controller in the prior art is constituted as above described, when a modulated wave accompanied by large amplitude variation is a control object, namely, when envelope of the modulated wave is not constant, a wide dynamic range is required for each component on the feedback loop. Also the time constant of the LPF 9 in the feedback loop must be made significantly large so as to remove influence due to the modulation and to control the mean power. Consequently, problems exist in that rapid rise of the transmission output required for burst mode transmission or the like in the TDMA system cannot be dealt with easily.